FEASIBILITY STUDY OF VARIABLE MULTILEVEL QAM MODEM FOR WIRELESS ATM NETWORKS

This paper describes a variable multi-level QAM modem applied to a wir eless ATM transport network with the aim of effectively offering ATM n etwork services over a terrestrial digital radio system. The concept o f the wireless ATM transport network based on a Virtual Path (VP) capa city control system which optimizes both the number of channels and th e multi-level QAM scheme for existing traffic variation is discussed. To achieve a hitless switch as a technical requirement of this network , we propose a modem configuration and a modulation scheme control (MS G) signal transmission. In this modem configuration, a multi-level con trol logic circuit in the modulator is used as the signal formatter. A modulated signal for the modulation scheme is maintained at a constan t average power. Decision data and error signal selection for the rece ived signal is carried out in the multi-level control logic circuit in the demodulator. The fluctuation of the demodulator loops due to modu lation scheme switch can be reduced by using a fully digitized AGC loo p and by converting the received signal to the condition of decision l evel constant. The MSC signal inserted into the first path data signal is transmitted without error by arranging the maximum amplitude of th e signal point set. In this way, switching between the modulator and t he demodulator is possible frame by frame. Finally, we present experim ental results for a variable multi-level QAM modem employing four modu lation schemes: QPSK, 16 QAM, 64 QAM, and 256 QAM. Through experiments , we prove that the modulation scheme is switched without fluctuation of the demodulator control loops by maintaining the signal condition o f the decision level constant. The achievement of a hitless switch for multi-level QAM is also confirmed by experiments.